1. Field of Invention
The present invention relates to a reduced-impedance chamber for use in plasma processing applications and more particularly to a chamber that can be used in repeatable and controllable plasma processing applications.
2. Description of Related Art
A conventional chamber, as shown in FIG. 1, includes a process chamber 10 having a chamber wall 12. A chuck assembly 14 is mounted on a bellows 13 that is mounted on a chuck mount ring 16. Chuck mount ring 16 includes spokes 18 through which chuck mount ring 16 is connected to chuck assembly 14. A workpiece, such as a semiconductor wafer 15, is mounted on chuck assembly 14. RF energy can be applied to chuck assembly 14 through a chuck impedance matching assembly 20. A plasma source 24 and an injection assembly 21 through which operational gases are injected into chamber 10 are above chuck assembly 14. A turbo-molecular pump 26 for evacuating operational gases is below chuck assembly 14. A gate valve 25 between chuck assembly 14 and turbo-molecular pump 26 provides selective isolation of turbo-molecular pump 26 from chamber 10 to enable detection of leaks by monitoring the leak up rate and to enable regulation of chamber pressure by varying the conductance to turbo-molecular pump 26. Coil 28 is provided in plasma source 24 to create a plasma therein. RF energy is supplied to coil 28 through fast match assembly 22.
The chamber illustrated in FIG. 1 operates as follows. First bellows 13 is lowered. Then wafer 15 is introduced through a slot valve (not shown) in the side of process chamber 10 usually below the operating position of chuck assembly 14. Wafer 15 comes in on a blade (not shown) which has slots to allow for typically three pins (not shown) in chuck assembly 14. The pins are able to move up and down by a mechanism internal to chuck assembly 14. Once wafer 15 is over chuck assembly 14, the pins lift wafer 15 off the blade, and the blade is then removed. After the blade is removed, the pins are lowered so that wafer 15 rests on chuck assembly 14, and bellows 13 is raised.
A relatively high DC voltage is then applied at chuck assembly 14 to fix wafer 15 to chuck assembly 14. Wafer 15 is electrically isolated from chuck assembly 14. In one common version, chuck assembly 14 is anodized, usually by a special process with additional post coating to improve dielectric properties. In a second version a conductive material is positioned between polyamide sheets. The conductive material receives the clamping voltage. Chuck assembly 14 is not held at the electrostatic voltage but is at the DC ground. The chuck achieves a potentially high negative DC voltage through self-bias and leakage current. Since the chuck is capacitively coupled, it can achieve a DC voltage.
Operational gases are injected into chamber 10 through injection assembly 21. RF energy is applied to coil 28 to create a plasma, and RF energy is applied to chuck assembly 14 through matching network 20 to generate a negative voltage on the wafer by means of self-bias. The self bias phenomenon results from the greater mobility of electrons as compared with the ions. For the ions to be drawn to the wafer surface at the same rate per RF cycle as the electrons, the wafer surface must generate the negative voltage. This is important for the process because it allows the ions to be accelerated to the wafer surface at an energy determined by the chuck RF voltage and the plasma parameters. After the process is completed, the injection of reactive gases is halted, the RF chuck power is removed, the wafer clamping DC voltage is removed or slightly reversed, the RF plasma energy is stopped, bellows 13 is lowered, and wafer 15 is removed.
If chuck assembly 14 is monopolar, then there is no means to deliver charge to wafer 15 through chuck assembly 14 during the clamping and unclamping of wafer 15. Wafer 15 must accumulate charge opposite to the chuck electrostatic electrode. The plasma is the most common means to complete the clamping circuit to wafer 15 since the plasma is a sufficiently good conductor even at low power levels. Usually the plasma is kept on at low power levels during clamping and unclamping operations.
The plasma completes a circuit path between the driven electrode at chuck assembly 14 (i.e., plasma cathode) and the typically grounded counter electrode (i.e., plasma anode). The counter electrode is usually injection assembly 21. In many systems, there are areas of the chamber wall that function as the counter electrode; if these walls are too close to the wafer, then they generate process uniformity problems or non-normal ion-accelerating electric fields. With the normal positioning of chuck assembly 14 in chamber 10, the counter electrode conducts to ground through the following path: (1) from injection assembly 21; (2) through plasma source 24; (3) along inner wall 12 of chamber 10; (4) through spokes 18 to the outer diameter of bellows 13; (5) through bellows 13; and (6) to the base of chuck assembly 14. The combination of chamber 10, bellows 13, spokes 18, plasma source 24 and injection assembly 21 represents a relatively high impedance as compared with the plasma bulk and sheath impedance.
FIG. 2 shows the equivalent circuit of the conventional chamber of FIG. 1. Chuck assembly 14, which is modeled by an inductor 100, a resistor 102, a capacitor 104, a capacitor 106 and an inductor 140, is adjacent to the position in the circuit corresponding to wafer 15. Proximate to wafer 15 are a capacitor 108 and a resistor 110, which are used to model the RF current that bypasses the sheath and heats the plasma. The rest of the model for the plasma includes a resistor 136, a resistor 138 and a current source 135, where current source 135 produces a current related to plasma parameters at wafer 15. As discussed above, the path to ground from injection assembly 21 is through plasma source 24, modeled by a capacitor 130, an inductor 132 and an inductor 134, wall 12 (including the electrostatic shield), modeled by a capacitor 124, an inductor 126 and an inductor 128, spokes 18, modeled by a capacitor 118, an inductor 120 and an inductor 122, and bellows 13, modeled by a capacitor 112, an inductor 114 and an inductor 116. An RF power supply 139, which is modeled as a voltage source 148 and a resistor 150, is connected to chuck assembly 14 through matching network 20, which is modeled as a capacitor 142, a capacitor 144 and an inductor 146.
All of these components contribute to the impedance in the ground path. Typically, the connection to the chuck can have an inductance of 50 nh and a capacitance to ground of 200 pf; the bellows can have an inductance of 250 nh and a capacitance of 100 pf, the spokes can have an inductance of 33 nh and a capacitance of 50 pf; the wall can have an inductance of 40 nh and a capacitance of 100 pf; and the plasma source can have an inductance of 40 nh and a capacitance of 100 pf.
In this model the plasma impedance is fixed. The plasma current source uses Langmuir probe data to determine the parameters of the current. These effects together produce harmonics. The generally nonlinear impedances of the elements described above result in the generation of higher order harmonics in the plasma voltage at the workpiece whereby the processing is difficult to control.
For the circuit in FIG. 2, FIG. 3 illustrates the predicted waveform of the voltage across the plasma sheath from the plasma anode to the plasma cathode. In this model the voltage across the bulk of the plasma is ignored. This voltage accelerates the ions to the wafer and controls the energy of the ions arriving at the wafer. The model assumes a perfect match for the fundamental of the RF energy at the input to the matching network 20. FIG. 4 shows the corresponding frequency spectra of the voltage across the plasma sheath for the conventional chamber. Here the fundamental frequency is approximately 13.6 MHz, the first harmonic is approximately 27.2 MHz, and the second harmonic is approximately 40.8 MHz.
The conventional chamber has a large variation in the harmonic content of the voltage across the plasma sheath when parameters of the system are varied. In high plasma densities with short sheath distances, Monte-Carlo simulations have shown that the ion energy upon arrival at the wafer is a strong function of the amount of these voltage harmonics at the wafer. This means the conventional chamber generates different ion energies under similar conditions when there are small variations in plasma density or coupling impedances in the system. Significant plasma density variations can be caused by small variations in pressure, power coupling to the plasma, or species. Likewise, coupling impedances can vary significantly because of manufacturing or refitting errors. Thus, the processing is not easily repeatable because of factors that may change from one processing sequence to another and from one chamber to another and thereby affect the level of the higher order harmonics. Such factors include:
1. Changes in the geometry of the RF components due to refitting;
2. Changes in the process pressure that change harmonic content;
3. Changes in the plasma source or chuck power that change the harmonic content;
4. Changes in the process gas species that result from changes in the process design;
5. Changes in the precise position of the match in the matching network(s).
More details of these physical processes are found in xe2x80x9cDynamics of Collisionless rf Plasma Sheathsxe2x80x9d by P. A. Miller and M. E. Riley (J. Appl. Phys., Vol. 82, pp. 3689-3709, Oct. 15, 1997); this article is incorporated herein by reference.
Conventional chambers for plasma processing and their use have been discussed elsewhere. Keeble (U.S. Pat. No. 4,844,775) describes an apparatus for use in treating semiconductor wafers by an active ion technique or by chemical vapor deposition. Flamm et al. (U.S. Pat. No. 4,918,031) describes a process for anistropic plasma etching utilizing a helical resonator operated at relatively low gas pressure. Savas (U.S. Pat. No. 5,534,231) describes a plasma reactor with RF power inductively coupled into the reactor chamber to produce an RF magnetic field substantially perpendicular to a pedestal on which a wafer is placed for processing. These three patents are incorporated herein by reference.
Accordingly, it is an object of this invention to provide a chamber with reduced impedance for plasma processing.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where chamber walls, bellows, and spokes of the chamber bottom are not included in the conductance path from the chuck to ground.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where the inject electrode dimension is increased so that the voltage drop across the plasma anode sheath is very low.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where the ions accelerated by the plasma sheath at the element to be processed are controlled with respect to changes in fittings and variations in substrates.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where the frequency spectra of the plasma voltage at the element to be processed is relatively insensitive to variations in plasma impedance.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where the ion energy at the element to be processed can be more controlled.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where an inject-exhaust plate may be designed to inject the process gas uniformly over the wafer.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where an inject-exhaust plate may be designed to allow for exhaust gas to be pumped through it without providing significant conductance loss.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where an inject-exhaust plate may be designed to be temperature controlled to reduce particle generation and surface reactions.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where an inject-exhaust plate may be designed to be of a material that will not react with the process.
It is a further object of this invention to provide a chamber with reduced impedance for plasma processing, where an inject-exhaust plate may be designed to provide low impedance in the RF circuit path.
The above and related objects of the present invention are realized by a system that includes a plasma source for generating a plasma in a chamber. A chuck assembly for mounting an element to be processed by a plasma forms a wall of the chamber. The chuck assembly is electrically connected to the plasma source and forms the plasma cathode. A plasma anode is electrically connected to the plasma source.
The plasma anode may desirably have a larger surface area than the plasma cathode. The plasma anode may be an inject-exhaust plate formed in a position opposed from the chuck.
A preferred embodiment of the present invention includes a chuck assembly upon which is mounted a workpiece. RF energy can be applied to the workpiece through the chuck assembly. The chuck assembly is electrically connected to a plasma source. The plasma source includes a coil and an electrostatic shield. An inject-exhaust plate is connected to the plasma source. The inject-exhaust plate separates the plasma source from a pumping plenum, a turbo-molecular pump, and perhaps a conductance controlling valve (e.g., a gate valve). The chuck assembly is connected to a chuck motion assembly through a bellows. A transfer chamber is also provided near the base of the chamber. When the bellows raises the chuck assembly to its working position, the chuck assembly seals with and becomes electrically connected to the plasma source.
A preferred embodiment of an inject-exhaust plate includes an array of holes as pumping ports and gas injects.
In a second preferred embodiment of the present invention, the inject-exhaust plate includes a driven electrode as part of the plasma source to capacitively couple energy into the plasma.